Means for and method of detection



p i 23, 1935- s. BALLANTINE 1,

- MEANS- FOR ANb IMETHODOF DETECTIQN" Orig inal Filed April 10, 1929Patented Apr. 23, 1935 UNITED sTATss MEANS FOR AND Ms'rnon F DETECTION:I

Stuart'Ballantine, ItIountain Lakes, N .1, assigner, by mesneassignments; to Radio Corporation of'America, New Yo'rk;N:Y.;acorporagtion of Delaware Original applicatioriApril-10,1929, Serial Nd.

354,022. Divided and this application Decem-- ber 20, 1929, Serial No.415,571;

6 Claims. (erase-27) 7 called heater? type. Aradio frequency, by'passThisinventionrelates to, an electrical circuit for thereception ofcarrierswave signals, and

more particularly to new and improved. forms of detector circuits andmethods of detection.

Objects of the present invention are to pro vide a detector and methodofdetection in which the response in the form of audio frequency. outputvoltage bears an approximately linear relation to; the input in the formof radio frequency carrier wave voltage. I

Further objects are to provide a vacuum tube detector" of the typehaving a limited range oi linear response and carrier wave amplifierscapable of adjustment to bringthe amplified signal voltage within, or tomaintain the (same within the limited range of linear response.

These and other objects of the invention; will be apparent. from thefollowing specification when taken ,withflthe accompanying drawing :inwhich Fig. 1 is a circuit diagramof a receiver embodying the invention,Fig. 2 is a curvesheet showing the variation, with carrier voltage, ofthe fundamental audio frequency response, the harmonic frequencies,and-the distortion, respectively, and Fig. 3 is a circuit diagram ofacornplete receiver which includes another embodi-a ment of theinvention. g

In the drawing, the numerals l, 2, indicate the terminals across whichthe signal energy is impressed, being transferred by the tuned radiofrequency transformer 3 to the amplifier tube t. Although but one,amplifier tube is illustrated, it will be understood thatany'appropriate number of amplifiers will beemployed to raise thereceived signal voltageito the desired order ofmagy nitude. Theamplified 'voltage'is transferred by the tuned transformer 5 to thedetector tube 6, the transformersecondary being shunted by, a voltagedivider l for adjustmentof the detector input voltage E0, as will behereinafter described.

' The audio frequency voltage Ea is developed acrossan appropriate'plateimpedance, such as the primary of an'audio frequency transformer 8, andthe secondary'is preferably tapped, as shown at 9, or otherwise providedwith adjustable means for'regulatingthe audio frequency energy impressedupon the single amplifier tube I!) which works into the loud speaker orother reproducer II. Y

As shown, the detector 6 is of thethree electrode type provided withacontrol grid G, anode or plate P and electron emittingcathode E whichmay, as is now wellunderstood, be of either the direct current oralternating current filamentary typ or may e. 0i the indirec l heaedprsw condenser C is connected between plate and fila-,

P TENT Y OFFICE??? I ment, and appropriate sources of grid bias and 7plate potentials E0 and Eb, respectively, are'provided to secure platecircuit rectification;

In such anarrangement-if arnodulate'd radio frequency signal of thecarrier-wave symmetrical side-band type, which maybe represented by:.

V e' E b(l+m sin at) sin wt, where 'Eo=impressed radio frequencycarrier'volt'ag'e m=coefiicient of modulation a=frequency of audiomodulation =angular velocity of the radio frequency current is impressedupo'ri the detector input terminals as indicated, and the output 'audiofrequency voltage across the plate impedance is observed and plotted asordinates against the radio fre quency carrier Voltage E0 as abscissae,a curve similar to that marked F in Figure 2 is obtained. The particularcurve F shown inFigure 2 was plotted from actual experimental dataobtained with a vacuum tube of radio [frequency "carrier voltage E0 issmall the detector, 'operateslon the curved portionyof the plate currentcliaracteristic,v and the response; as determined by audio frequencyoutput voltage,

is proportional to the square ofthe impressed carrier voltage." Thus,the slope of curve 3', up

scale. The relationshipbetween signal voltage the 20l-A type," usinglogarithmic coordinates; 'When the impressed andjharmonic'sf isrepresented graphically by curve H of" Fig. 2;

In the prior artthe detector has ben operated at low signal voltages. Inthe conventional broadcast, receiver comprising a grid-rectifyingdetector succeeded by a two stageaudio amplifier, the normal output, asdefined by the Standardization Committee of the Institute offRadioEngineers, "is 20 v olts across a resistance'of 5000 ohms in the platecircuit of the output tube. For? a mu of 5' and internal resistance of5000 ohms in this tube this would correspond to 8 volts on the grid, andfor an amplification of about persta'ge this wouldcorrespond ,to 8/625or 0.013 volt vaudio on the detectorgrid. If the average modulation is25% the corresponding radiofrequency carrier-vo1tage would be about so,-I

023- volt. Thisis Well within the small signal I region in which thedetector" responds according to the square-law. It is also to be notedthat in a conventional receiver of this type it will not be possible toget out of the square-law region because with this amount of audioamplification the output tube will overload before the carrier voltageon the detector grid has increased suificiently to do so. In order toget out of this region it is generally necessary to reduce the audioamplification, for example, by removing one audio stage, as hereinafterdescribed.

The disadvantages of such square-law detection have been discussed inUnited States Patent No. 1,698,668, issued January 8, 1929,"toBallantine and Hull. Briefly with a modulated carrierwave signal of thetype e Eo(l+F(t) )sin wt the output of such a detector contains not onlya term proportional to F(t) as it should, but also a second order F 6)term which is superfluous and represents distortion. If F(t) is of theform m sin at where m is the degree of modulation (0 to 1.0, or 0 to100%), the distortion will be represented by a second harmonic, ordouble frequency term. If we define the distortion as the ratio: I

i L M where En represents the amplitude of the nth harmonic and E1 isthe amplitude of the fundamental the distortion in the case ofsquare-law detection with the above modulation is equal to In order tokeep down this distortion the degree of modulation m must be limited,that is m should be kept small compared with unity. This is uneconomicaland wasteful of power in the carrier, and limits the service area of agiven broadcast transmitter for fixed interference production by thecarrier. In'the early days'of broadcast transmission the transmitterswere seldom capable of greater than 50% modulation. In English speechthe ratio of the peak voltage to the average voltage is about 5, henceif the peaks are limited to 50% the average modulation will be about10%. With such low modulation the distortion might. not be noticeable,and this would account for the complacent attitude with which theprocess of square-law detection has been viewed'by experts in the priorart. With 100% modulation, however, these detectors are not satisfactorybecause the distortion is too great. I have therefore sought a' detectorwhich will respond, not as the square but asthe first power of theapplied voltage, in order to provide satisfactory reception withcomplete (100%) modulation. v

In the previous UnitedStates patent to Ballantine and Hull, cited above,it is shown how with a rectifier which has an EI characteristiccomprising two slightly curved branches meeting in a section of greatercurvature, an approximation to linear detection can be achieved byincreasing the signal voltage applied to the device. In the present caseof a thermionic device'we do not have two nearly straight branches, butwe dohave at least one perfectly straight branch, i. e., the part of thecurve of zero current along the axis for retarding fields. I havediscovered that with such a device, having at least one straight branchthe response approaches linearity as the signal voltage is increased andmore of the straight portion is utilized during the cycle.

The experimental proof of this is contained in Fig. 2, in which theslope of the response curve at points A and B is shown by broken linesa, b, respectively. As the applied carrier voltage is increased beyondthe region in which the detection is according to the square-law, thatis beyond the point A, the slope of response curves gradually decreasesuntil at the point B the slope of the curve is unity, indicating linearresponse. I propose to take advantage of this fact by amplifying andadjusting the signal'voltage so that operation takes place at thispoint, thereby obtaining minimum distortion.

In making the foregoing curves a modulation of 20% was employed, and itwill be seen that for small signal voltages the distortion approaches asshown at the left-hand end of the distortioninput voltage curve D. As E0increases, however, the distortion diminishes when the exponent in thelaw of response stated above begins todecrease from 2 to 1. Thus at thevoltage E0 shown at B, at which point the response is linear, thedistortion is a minimum, and is seen to have decreased from 5% to 57%.For higher values of impressed carrier voltage than those indicated atB, i. e., in the regionof overloading, the distortion increases rapidly,and attains values'which are in fact much higher than the distortionfor" small signals, the maximum value of the distortion curve in Fig. 2being 50%. Y 7

I have found that there is no practical difiiculty in adjusting thesignal voltage E0 to the optimum point B by manual control such as thevoltage divider 7; nevertheless it is desirable in apparatus designedfor commercial use to make the adjustment for linearity as easy aspossible. In my copending application, Ser. No. 354,022, filed April'10, 1929, of which the present application is a division, I havedescribed and claimed a method of extending the range of carrier voltageover which the response of the detector is approximately linearr I InFig. 3, I have shown a receiver system in which the carrier voltage maybe adjusted and maintained at its optimum value'by an automatic volumecontrol such as described in my co pending application, Ser. No.231,273, filed Nov. 5, 1927. As in the receiver shown in Fig. 1, thereis no audio amplifier stage between the detector and the power tubesince relatively high voltages are to be applied to the detector and thepower tube would be greatly overloaded if the conventional audioamplifier system were employed.

The radio frequency amplifier is represented by a single triode stage;this is symbolic only. L1 L2 represent the input transformer tuned bythe condenser C1; '1: is the detector tube arranged for platerectification and is here shown as a triode; C4 is a by-pass condenserfor radio-frequency currents; T is an audio frequency transformer; T3 isa power tube which supplies. power to the electrophone LS.

The automatic volume control which, in accordance with this invention,is to be adjusted to effect operation of the detector T2 within therange of substantially linear response, comprises a triode rectifiertube T4 which is actuated, through the isolation condenser C3, by thesignal impressed upon the detector. The plate circuit of the controltube T4 includes a resistance R2 and high tension battery E1, the directcurrent potential drop across resistance R: being impressed as a gridbias upon the radio frequency amplifier tube T1 while the modulationfrequencies are by-passed by condenser C5. With this arrangement, thebias voltage upon the radio frequency amplifier will vary automaticallywith the strength of the received signal and, by a suitable choice ofthe circuit constants, the grid bias and thereby the amplification oftube T1 may be so controlled that the amplified signal voltage impressedupon the detector may be maintained substantially constant. By means ofthe adjustable bias voltage E2 and grid leak R1, the operation ofcontrol tube T4 may be adjusted to any carrier voltage level, and, inaccordance with this invention, the adjustment is set at the carrierwave voltage corresponding to the linear response range of the detector.

The output from the reproducer LS is adjusted, after setting theautomaticrvolume control for linear detection, by adjustable controlmeans S1.

The circuit arrangements herein described show certain embodiments of myinvention and certain methods of operation for the purpose of explainingits principle and showing its application, but it is obvious to thoseskilled in the art that there is a great variety of arrangements whichmay be employed for producing the desired result and I aim therefore tocover all such modifications and variations. I

I claim:

1. In the operation of a detector of the type having a restricted rangeof linear response lying outside of the range of square-law response andimmediately short of the point of overloading, the method of securinglinear response which comprises amplifying received signal energy,impressing the amplified energy upon the detector, and adjusting theamplification rate to bring said amplified energy Within the restrictedrange of linear response. V

2. The invention as set forth in claim 1 wherein the rate ofamplification is automatically controlled by the magnitude ofzthe signalenergy to maintain the amplified" signal energy impressed upon saiddetector within the limited range of linear response. r Y

1 3. In the reception of modulated carrier wave signals with amplifiersand a detector of the vacuum tube type, said detector having arestricted range of linear response lying beyond an extended range ofsquare law response and immediately short of the point of overloading,the method of reducing distortion which comprises amplifying receivedsignal voltages to values in excess of those corresponding to square-lawresponse of the vacuum tube detector, impressing said'amplified signalvoltages upon the detector, and impressing upon the last amplifier stageof the receiver the unamplified audio frequency voltages developed inthe detector stage.

4. In a carrier wave receiving system, the combination with a detectorand associated elements effective to secure, over an approximately pointrange of impressed carrier voltages, a linearrelation between outputcurrent and carrier voltage, of means for amplifying received signalvoltages to values within and in excess of said range,

andmeans for adjusting the amplified signal voltage impressed upon saiddetector to fall within said range.

5. In a radio receiving system, the combination with a detector stagecomprising a vacuum tube, of the type having a restricted range oflinear response lying beyond an extended range of square law responseand immediately short of the point of overloading, circuit elements andmeans for applying to said tube energizing potentials effective tosecure a linear response over a range'of carrier voltages, of anamplifier for amplifying received signal energy, and means automaticallycontrolling the amplification of saidamplifier to maintain thevalue ofthe amplified signal voltage impressed upon said-vacuum tube detectorwithin the said range of linear response.

' 6. In a radio receiving system, the combination of a vacuum tube andcircuit elements-associated therewith to form a detector stage, saidtube having a limited range of linear response lying between an extendedrange of square law response and the point of overloading, an outputtube coupled to saiddetector and actuated by the unamplified audiofrequency response developed in said detector stage, of a vacuum tubeamplifier having a maximum gain adapted to amplify received signalvoltages to values in excess of those corresponding to square-lawresponse of said detector, and means automatically adjusting the gain ofthe vacuum tube amplifier as a function of received signal energy tomaintain the detector input within that voltage range for which saiddetector has a linear response.

STUART BALLANTINE.

